Dynamic Random Access Memory or DRAM uses capacitors to store bits of information within an integrated circuit. Some DRAM devices use Metal-Insulator-Metal or MIM capacitors. MIM capacitors in DRAM applications use insulating materials with a dielectric constant higher than that of SiO2 (3.9), such as HfO2 and ZrO2, commonly referred as high K dielectric materials. Dielectric constant, or K value, is a measure of a material's ability to be polarized; polarization is closely associated with a material's ability to hold electrical charge. Therefore, the higher the dielectric constant of a material, the more electrical charge the material can hold. A capacitor's ability to hold electrical charge (capacitance) is a function of a surface area of the capacitor plates, a distance between the capacitor plates, and the dielectric constant of the insulator. Capacitors made with high K materials can be made smaller than more conventional capacitors with equivalent capacitance. Reducing the size of capacitors is important for reducing the size of integrated circuits.
As DRAM technologies scale down below 40 mm (referring to the average half-pitch of a memory cell, or half the distance between cells in a DRAM chip), manufacturers must reduce the equivalent oxide thickness of dielectric films in MIM capacitors to increase charge storage capacity. Equivalent oxide thickness or EOT is a measure of thickness of a film of silicon oxide would have to be to achieve the same effect as a film of a high K dielectric material. Manufacturers typically achieve lower equivalent oxide thickness by reducing physical dielectric film thickness.
Reducing physical film thickness leads to increased leakage current in MIM capacitors (a phenomenon where current passes through an insulator, compromising storage capacity). DRAM applications utilize MIM capacitor stacks, also known as MIM capacitors with leakage current below 1E10−7 A/cm2. Leakage current in MIM capacitors using high K dielectric materials is typically dominated by either Schottky emission or Frenkel-Poole emission. Schottky emission, also called thermionic emission, is the heat induced flow of charge over an energy barrier. The effective barrier height of some MIM capacitors using high K dielectric materials with narrow energy band gaps (such as TiO2 and Nb2O5) controls leakage current due to Schottky emission. Effective barrier height is a function of the difference between the work function of the electrode and the electron affinity of the dielectric. The leakage current of some other MIM capacitors using high K dielectric materials with wide energy band gaps (such as ZrO2 and HfO2) is dominated by Frenkel-Poole emission, which is related to leakage conduction through charge traps in the energy band gap. In either case, the leakage current of MIM capacitors can be reduced by introducing proper dopants.
Current MIM capacitors used in DRAM applications use HfO2 or ZrO2 as the insulating dielectric material. Manufacturers commonly dope HfO2 and ZrO2 with oxides that have higher conduction band offset to increase the barrier height, and/or dope the dielectric material with acceptor-type dopants to neutralize the charge traps in the dielectric material. The conventional doping method for high K materials is to dope tetravalent oxides, such as TiO2, HfO2 and ZrO2 with trivalent oxides, such as Al2O3 and Y2O3. Some emerging materials such as TiO2 have still higher K values, but lower conduction band offset than lower K materials, producing higher leakage current.
Doping TiO2 with Y2O3 or Al2O3 can reduce leakage current to approximately 5E10−7 A/cm2 with equivalent oxide thickness of 0.5 nm. However, reducing leakage current further by tuning the doping concentration or layering structure is difficult.